A Guide to How Chainsaws Work

by:Jiali     2020-04-20
The modern chain saw actually has a lot in common with a Formula 1 car. That is, they both have lightweight engines with incredibly high power-to-weight ratios and their composite-construction bodywork is attached to the engine which is the main structural member. Of course where the comparison fails is if you choose an electric machine, but for now let's look at the element that is common to all chainsaws: the cutting or business end. The Cutting Edge The basic principle is that small cutting blades are made to circulate around a guide bar. They cannot be large or they would bind and stall from contact with the wood. The chain needs to be slightly loose on the bar to aid easy running and to allow for the heating up during operation, which swells the parts and can jam up the chain. Oregon of the USA pioneered the modern bar and nowadays claim an anti-vibration bar and chain design: they are used by chain saw makers like Draper. The actual angled, curved blades are made in chrome-plated steel and they cut from their top and side leading edges. They alternate between left and right-angled blades. They are held together with rivets which allow them to articulate when going around the bar. Tungsten carbide blades are available for difficult work such as roots, where contact with earth and stones may occur. Chains are described by their length and by their pitch (half the length of three linking rivets, e.g. 3/8') as well as their gauge, (the thickness of the drive link that goes into the groove of the bar, e.g. 0.05'). The drive link hooks up with the sprocket from the engine to turn the chain, and also functions as a means whereby the chain oil is distributed around the bar and to the blades. Modern saws have a small pump to automatically oil the chain from the oil tank. The chain oil is deliberately thick and tacky to enable it to resist being thrown off the blades. This is why using regular motor oil should only ever be a temporary fallback because it is too thin and will not protect the blades and bar as efficiently. In front of each blade on a regular saw is a depth gauge: this controls the depth of the cut and is a safety feature. If the gauge is too high, poor cutting will result: but file it too low and you can get the dangerous kickback phenomenon. The Engine End Looking briefly at the electric motor, this is essentially an electromagnet, the armature with its coils of wire, in a field being created by a field magnet around it, and being rotated by having its polarity endlessly reversed. Through the armature is an axle with a circular commutator mounted around it like a sleeve. This is kept in contact with sprung metal brushes, usually with carbon heads, that supply the electric current. Brushes are the obvious items to wear out and so chain saw makers usually supply a spare pair. If you get more serious damage such as a burn-out armature or failed bearings, the low cost of replacement of the machine often makes repair an expensive option by comparison. The 2-stroke petrol engine, however, is a different matter. This is an air-cooled alloy machine with cooling fins on its body; no water pump or radiator. It is a masterpiece of construction in that it can weight less than 2kg dry. It does not require cams or valves and its combustion cycle includes firing on every rotation instead of every second, meaning that in theory it can be twice as powerful as a comparable 4-stroke. The single piston in the combustion chamber is connected by a con rod to the crankshaft, which runs on roller bearings. It is supplied from the fuel tank with a pre-mixed fuel-oil mixture which feeds the engine and lubricates the moving parts. This mixture is admixed with air by the carburetor. Its throttle plate controls the amount of air by opening and closing its flap. The air is drawn in by the venturi, a restriction in the diameter of the tube that creates a vacuum. Fuel is drawn in by vacuum through the jet of the carburetor. There are adjustment screws for the correct idling speed and throttle. The choke obstructs air to enrich the mix going into the engine when starting. It must be turned off when running to avoid an over-rich mixture that deposits carbon on the plug and produces smoke and cut-outs. A pull-cord starter uses 2 pawls that are spring-loaded and only engage on the drive when the cord is yanked. They are mounted on the aluminium flywheel (which has external blades to cool the engine, such is the economy of function and space in this design). On the edge of the flywheel are built in two magnets, which create a magnetic field when rotating. This powers the magneto, whose armature induces a small electric current and the magneto's 2 coils compress that into an ultra-high voltage spike (c. 20,000V) that feeds through to the head of the spark plug. The ignition of the mixture by the spark expands the air in the chamber and pushes down the piston, thus creating circular motion through the crankshaft.. Exhaust takes place through the spark arrestor which doubles as a silencer. Drive is supplied through a centrifugal clutch. This is not engaged when the engine is idling, so the chain does not turn. When you apply throttle, the faster engine speed pushes the two clutch plates outwards, thus engaging the drive. It is automatic and low maintenance. A sprocket mounted on the drive connects to the chain, which on modern models from makers like Makita is often supplied with an easy-change removal system that does not require socket spanners. So what you may regard as a saw with some power attached is more like a very cleverly designed and packaged power plant with a few blades hung off it.
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